As is known, limiting amplifiers are widely used in optical communications. A limiting amplifier receives an input signal presenting a stream of binary data, amplifies the input signal into saturation with a high gain and outputs a substantially two-level output signal exhibiting the binary data. Limiting amplifiers preferably have a high gain so that they can amplify the input signal into saturation. At the same time, a limiting amplifier should have a sufficient speed to keep up with a rapid change in the input signal. In addition, as is common in many high-gain amplifiers, an offset at an input of a limiting amplifier needs to be properly handled; otherwise the offset may be amplified to an extent that the offset saturates a latter stage of the limiting amplifier regardless of the input signal.
A conventional limiting amplifier 100 is depicted in FIG. 1. Limiting amplifier 100 comprises seven amplifier stages 110-170, each receiving power from a power supply node VDD and grounded to a ground node VSS, for receiving a differential input signal comprising two ends Vi+ and Vi (hereafter Vi+/Vi) and outputting a differential output signal comprising two ends Vo+ and Võ(hereafter Vo+/Võ). Limiting amplifier 100 further comprises a low pass filter (LPF) 190 for receiving the output signal Vo+/Võ and outputting a differential filtered signal comprising two ends Vf+ and V{tilde over (f)}(hereafter Vf+/V{tilde over (f)}), and a feedback amplifier 180 for receiving the differential filtered signal Vf+/Vf and outputting a differential signal into two common nodes 111 and 112 that are shared with a first amplifier stage 110. The seven amplifier stages 110-170 provide a high gain to the differential input signal Vi+/Vi. However, there might be an offset at the input of first amplifier stage 110 that is also amplified by the high gain. LPF 190 comprises a pair of R-C network (191-192 and 193-194) to extract a low frequency component of the differential output signal Vo+/Võ that primarily originates from the offset.
As a result, the differential filtered signal Vf+/Vf− is basically an unwanted component of the output differential signal Vo+/Vo. Feedback amplifier 180 receives the differential filtered signal Vf+/Vf−, amplifies the differential filtered signal Vf+/Vf−, and transmits the amplified output into the two nodes 111 and 112 with a polarity reversal. A negative feedback loop comprising amplifier stages 120-170, LPF 190, and feedback amplifier 180, is thus formed. Due to LPF 190, the negative feedback is effective only for the low frequency component of the differential output signal Vo+/Vo− that primarily originates from the offset at the input of the limiting amplifier 100. Due to the high gain nature of the negative feedback loop, the unwanted offset is effectively suppressed. The feedback amplifier 180, however, increases loading at circuit nodes 111 and 112, suppresses the amplification function of the first amplifier stage 110, and slows down the overall speed of limiting amplifier 100.
What is desired is a limiting amplifier that utilizes a feedback scheme that does not slow down the overall speed of the limiting amplifier.